The current mode of transporting liquid media, such as chemicals, oil and agricultural products, is mainly in tankers which are equipped with rectangular cargo tanks that are integral with the ship, so-called parcel tankers. The cargo tanks are part of the ship's structure, in which the tank walls are formed by the ship's hull, profiled cross bulkheads and longitudinal bulkheads placed therein, and the ship's deck.
The disadvantage here is that cracks can occur in the tank walls through deformations of the ship in heavy seas and through temperature differences. The abovementioned deformations cause high stress concentrations in the tanks, particularly on the corner points, which can result in the formation of cracks. If this happens, an opening can develop between two adjacent tanks, with the result that undesirable mixing of the stored products can occur. In the current regulations it has already been stipulated for many products that adjacent tanks must not be filled with different products, this being to prevent the risk of cross-contamination and to avoid a dangerous situation. Owing to the fact that different products can be transported in the tanks, the tanks have to be carefully cleaned after delivery, in order to ensure that a product to be transported after that is not contaminated. However, the tanks are difficult to clean. This is partly because of the fact that the walls are of a partially profiled design in order to make them sufficiently rigid, and the fact that they have corner points. This means that a relatively large quantity of flushing water is needed for cleaning the tanks, which is expensive and undesirable from an environmental point of view because the flushing water sometimes has to be discharged as chemical waste. In addition, a slight degree of contamination remaining in the tank cannot always be detected by a routine check, with the result that damage can occur to the products subsequently transported. Owing to the fact that the tanks are more difficult to insulate, greater temperature differences can occur in the stored products. It is also necessary to heat to a higher temperature in order to be able to maintain a desired temperature in the tank. The higher temperatures can cause deterioration of the product.
In the art a search has been going on for alternatives for quite some time now, one idea being, for example, to place several cylindrical storage tanks in the ship's hull. See, for example, U.S. Pat. No. 6,167,827 or DE-U-93.09.433.
Further, a is known from NL-C-1011836. This publication discloses a ship with a cylindrical transport tank placed in the ship's hull. In this case the bottom of the tank is supported on the ship's hull and is connected to a cylindrical tank circumferential wall. Spring means are provided between the lower side of the tank circumferential wall and the ship's hull. The spring means serve to limit a movement of the tank circumferential wall upwards and downwards. This means that cargo in the transport tank is supported via the bottom of the tank directly on the ship's hull, while the tank circumferential wall can move slightly relative to the ship's hull within the limits formed by the spring means.
It is a disadvantage here that the tank circumferential wall has to be of a relatively thick-walled design. Furthermore, it is a disadvantage that a relatively heavy tank roof is necessary. As a result of this, the total weight of the transport tank is relatively high. The possibilities for scaling up are limited, while the spring means are fragile and require maintenance. Deck passages for the tank have to be flexible.